Tailoring Interface Energies via Phosphonic Acids to Grow and Stabilize Cubic FAPbI3 Deposited by Thermal Evaporation

Coevaporation of formamidinium lead iodide (FAPbI3) is a promising route for the fabrication of highly efficient and scalable optoelectronic devices, such as perovskite solar cells. However, it poses experimental challenges in achieving stoichiometric FAPbI3 films with a cubic structure (α-FAPbI3). In this work, we show that undesired hexagonal phases of both PbI2 and FAPbI3 form during thermal evaporation, including the well-known 2H-FAPbI3, which are detrimental for optoelectronic performance. We demonstrate the growth of α-FAPbI3 at room temperature via thermal evaporation by depositing phosphonic acids (PAc) on substrates and subsequently coevaporating PbI2 and formamidinium iodide. We use density-functional theory to develop a theoretical model to understand the relative growth energetics of the α and 2H phases of FAPbI3 for different molecular interactions. Experiments and theory show that the presence of PAc molecules stabilizes the formation of α-FAPbI3 in thin films when excess molecules are available to migrate during growth. This migration of molecules facilitates the continued presence of adsorbed organic precursors at the free surface throughout the evaporation, which lowers the growth energy of the α-FAPbI3 phase. Our theoretical analyses of PAc molecule–molecule interactions show that ligands can form hydrogen bonding to reduce the migration rate of the molecules through the deposited film, limiting the effects on the crystal structure stabilization. Our results also show that the phase stabilization with molecules that migrate is long-lasting and resistant to moist air. These findings enable reliable formation and processing of α-FAPbI3 films via vapor deposition.

. shows the X-ray diffraction (XRD) pattern of a film deposited on FTO, revealing the presence of the 2H FAPbI 3 polytype, as evidenced by the presence of the characteristic peak of the (100) plane at 2θ = 11.65°.Additionally, a small amount of the cubic phase, α-FAPbI 3 , was detected with a peak observed at 2θ = 13.9°.However, a peak at 2θ = 12.5° exhibited the highest intensity, suggesting that another non-perovskite phase constitutes the majority of the film.This peak could potentially be associated with either PbI 2 or other hexagonal polytypes, such as 4H and 6H.Peaks from the 6H-PbI 2 phase match with the positions of the peaks marked with an *.However, precise phase identification is difficult in polycrystalline thin films due to the overlap of the peaks between all the phases and the low signal of the peaks at higher angles.Figure S10.The growth energy,  g (eV/Å 2 ), for PAc-capped PbI 2 -terminated FAPbI 3 versus the grown thickness (ℎ) obtained via eq(S4).The energy terms for each capped slab model are provided in Table 1.

Surface chemistry discussion of FAPbI3 and FAI films deposited on surface treated and bare FTO
To further understand the proposed mechanism of PPAc molecules migrating during thin film growth, we studied the surface chemistry of perovskite films grown on treated samples and bare substrates.In general, having a ratio of less than 3:1 for iodine:lead, and less than 1:1 for the organic:lead (often obtained from the nitrogen signal in XPS) is an indication of forming nonperovskite phases.In Figure S15, we present the ratio of I:Pb and N:Pb as a function of PAc concentration, using PPAc as an example, as it is the most effective molecule driving the conversion of the deposited FAPbI 3 film to the α phase.A stoichiometric α-FAPbI 3 is expected to have an I:Pb ratio of 3:1 and an N:Pb ratio of 2:1.The XPS data of films deposited on bare FTO reveals an I:Pb ratio around 3:1, as expected in a FAPbI 3 film, but a N:Pb ratio of about 0.9:1.This finding suggests that the films deposited on bare FTO have a deficiency of FA + cations.We speculate that this deficiency is contributes to the formation of the observed hexagonal phases.This is in addition to our findings that without capping FAPbI 3 with phosphonic acids the growth energy of the δ-FAPbI 3 is more favorable than that of the α-FAPbI 3 (Table 1).On the other hand, as the concentration of PPAc increases, the ratio of N:Pb also increases, achieving a value close to 2 in films where only the α phase can be observed in the diffraction pattern.For concentrations above 5 mM of the PPAc molecule, we found ratios above 2:1 of N:Pb.This high level of organic cations correlates with the emergence of lower dimensional phases in the film observed in XRD (Figure S14).Moreover, the O 1s XPS signal increases as we increase the PPAc concentration (Figure S15), suggesting the adsorption of oxygen species as more FA, which is hygroscopic 38 , is incorporated in the film.It is possible that this signal belongs to the oxygen species in the PAc molecules.In addition, we expect the excess of organics in FAPbI 3 films to induce the formation of low dimensional phases, analogously to what is observed in thermally evaporated MAPbI 3 perovskites 11 .Conversely, we observed no change in the I:Pb or N:Pb ratios of measured XPS for FAPbI 3 films deposited on the washed substrates, suggesting that a monolayer of PPAc is not enough to drive sufficient adsorption of FA molecules.
In addition to the FAI C=N bond peak signal (at 400 eV), the XPS N 1s scan shows the presence of a C-N bond peak (at 401.85 eV, Fig. S16a) in the films on substrates that are functionalized with PAc molecules.It is possible that this signal belongs to FAI byproducts with C-N bond characteristics and that are incorporated in the perovskite structure during deposition.However, no clear structural effects were shown as this XPS peak increases in intensity.

Figure S1 .
Figure S1.Circular integration of GIWAXS pattern of FAPbI 3 evaporated on bare FTO (black trace) and comparison with simulated patterns for the possible phases.

Figure S3 .
Figure S3.XRD pattern of as-deposited FAPbI 3 film of bare FTO

Figure S4 .
Figure S4.CL-SEM intensity maps and point CL spectra of co-evaporated films on bare FTO

Figure S6 .
Figure S6.UV-VIS spectroscopy of FAPbI 3 films on bare FTO annealed by varying time.

Figure S7 .
Figure S7.XRD patterns of FAPbI 3 films on FTO functionalized with different functional groups on the R-ligand of the phosphonic acid and concentration of 5mM.The peaks for PPAc and EPAc match closely those of the α-FAPbI 3 at around 2θ= 13.9° whereas the rest of the PAc molecules produce a combination of both α-FAPbI 3 and hexagonal phases, with peaks at 2θ= 12.5° and 11.6°.

Figure S8 .
Figure S8.Proposed capping mechanism of FAPbI 3 via phosphonic acids.Ball and stick models of PPAc (a, c, e, g) and 3PPAc (b, d, f, h) phosphonic acids capping FAI-terminated slabs of α-(upper panel) and 2H-FAPbI 3 (lower panel) both via PA (a, b, c, d), CH 3 of PPAc (e, g) and COOH of 3PPAc (f, h) are shown.Dashed lines indicate H-bonds between H of terminal FA + and O of phosphonate (a, b) as well as O of carboxyl group (d, h).O, C, N, H, P, Pb and I atoms are depicted as red, brown, light blue, white, lavender, grey and magenta spheres.For clarity, only the top few layers of the slabs are shown.

Figure S9 .
Figure S9.Proposed capping mechanism of FAPbI 3 via phosphonic acids.Ball and stick models of PPAc (a, c, e, g) and 3PPAc (b, d, f, h) phosphonic acids capping PbI 2 -terminated slabs of α-(upper panel) and 2H-FAPbI 3 (lower panel) via PA (a, b, c, d), CH 3 of PPAc (e, g) and COOH of 3PPAc (f, h) are shown.O, C, N, H, P, Pb and I atoms are depicted as red, brown, light blue, white, lavender, grey and magenta spheres.For clarity, only the top few layers of the slabs are shown.

Figure S11 .
Figure S11.CL-SEM intensity maps and point CL spectra of co-evaporated films on 3mM PPac

Figure S13 .
Figure S13.XRD patterns of FAPbI 3 films deposited on EPAc functionalized substrate with varying concentration.

Figure S14 .
Figure S14.XRD patterns of FAPbI 3 films deposited on PPAc functionalized substrate with varying concentration.

Figure S15 .
Figure S15.XPS analysis for Perovskite films deposited on substrates functionalized with different concentrations of PPAc molecules.(a) N 1s scan.(b) O 1s scan.(c) I/Pb elemental ratio.(d) N/Pb elemental ratio

Table S1 .
The energy corresponding to the first ligand binding the FTO surface, at PBEsol+D3 level of theory, for PA, CH 3 (of PPA) and COOH (of 3PPAc) binding modes.